Question

Rank imidazole, pyrrole, and benzene from most reactive to least reactive toward electrophilic aromatic substitution.

Short answer

The typical reactions of benzene ring are electrophilic substitution reaction just as the typical reaction of alkenes is electrophilic addition reactions.This reaction is initiated by an attack of an electrophile to the aromatic ring which provides two electrons to the electrophile to form a new covalent bond resulting in the formation of resonance stabilized carbocation called benzonium ion. In the next step, the base abstracts the proton from intermediate carbocation and the two electrons of the C-H bond goes back to the ring so that stable aromatic ring is generated. For example,nitration of benzene.

Step-by-step solution

Step-by-Step Solution  Step 1: Electrophilic aromatic substitution reaction

The typical reactions of benzene ring are electrophilic substitution reaction just as the typical reaction of alkenes is electrophilic addition reactions.This reaction is initiated by an attack of an electrophile to the aromatic ring which provides two electrons to the electrophile to form a new covalent bond resulting in the formation of resonance stabilized carbocation called benzonium ion. In the next step, the base abstracts the proton from intermediate carbocation and the two electrons of the C-H bond goes back to the ring so that stable aromatic ring is generated. For example,nitration of benzene.

Order of compounds   from most reactive to least reactive

Order of imidazole, pyrrole, and benzene from most reactive to least reactive towards electrophilic aromatic substitution are shown below:

Here, pyrrole and imidazole are both more reactive than benzene because both pyrrole and imidazole reacts with an electrophile to form a carbocation intermediate that can be stabilized by the resonance electron donation into the ring by nitrogen atom, whereas benzene doesn’t have any electronegative atom in the ring to stabilize the carbocation intermediate.

Pyrrole is also more reactive than imidazole, because the second nitrogen atom of imidazole is not able to donate the electrons to the ring by resonance, but can withdraw electrons from the ring inductively.